1. Field of Invention
The invention relates to a light-emitting diode comprising a chip capable of emitting visible light of a first wavelength, a light-emitting surface and a phosphor layer which is provided on the light-emitting surface and which is capable of converting light of the first wavelength to visible light of a second wavelength.
2. Description of Related Art
Such a light-emitting diode (LED) is well known. In the case of an LED, the chip emits colored light of one specific wavelength. Hitherto, LEDs were predominantly used as signal lamps, but as a result of the increased light output, LEDs are increasingly used, at present, to illuminate objects. In many applications there is a trend towards illuminating an object with light of a different color, particularly light comprising different wavelengths, for example substantially white light, which is the reason why the light-emitting surface of the known LED is provided with a phosphor layer which partially absorbs the light (of a short wavelength) which is emitted by the chip and converts said light to light of a different (longer) wavelength. By providing the phosphor layer in a thickness such that only an accurately determined part of the light emitted by the chip is converted to said other color, the desired result, for example substantially white light, can be achieved as a result of the fact that both colors mix.
The above-described LED has the disadvantage that in order to obtain a light output having a predetermined, accurate color temperature, the thickness of the phosphor layer must be very accurately controlled, which is a complex process. As a result, in practice, variations in the thickness of this layer occur, which may lead to undesirable color deviations, which may range from several thousand to several hundred Kelvin in color temperature.
WO 98/39805 discloses a light-emitting diode whose light-emitting surface is surrounded by a transparent mass comprising a mixture of three different types of phosphor emitting, respectively, blue, green and red light. By accurately controlling the composition of the phosphor mixture, light of the desired color can be obtained. A drawback of this diode resides in that a large quantity of the phosphor mass must be cast around the chip because all the (ultraviolet) light emitted by the chip must be converted to visible light.